A rotational angle detection device of non-contact type using an MR sensor is known that is used to detect a rotational angle of a rotating shaft as a detection object, as disclosed in, e.g., Patent Document 1.
Here, a conventional rotational angle detection device of this type will be described below with reference to FIGS. 9 and 10.
As illustrated in FIG. 9, a rotational angle detection device comprises: a rotating magnet 102, which is attached to a rotating shaft 101 as a detection object and rotated with the rotating shaft 101; and a magnetic sensor 103, which is positioned in the vicinity of the rotating magnet 102, without contacting with the rotating shaft 101.
The magnetic sensor 103 comprises a pair of sensor units 121 and 122, each with bridge-connected four MR elements 111, respectively, as illustrated in FIG. 10, etc. Each of the four MR elements 111 that configure each of the sensor units 121 and 122 have magnetic anisotropy as their physical attribute, respectively. That is, the MR elements 111 provide different resistance values in magnetic fields along a easy-magnetization axis from those in magnetic fields along a difficult-magnetization axis perpendicular to the easy-magnetization axis.
The four MR elements 111 that configure the sensor units 121 or 122 are positioned in such a way that their easy-magnetization axes are displaced by an angle of 90° with respect to each other. In addition, each MR element 111 in the sensor unit 121 and each MR element 111 in the sensor unit 122 are positioned in such a way that their easy-magnetization axis are tilted by an angle of 45° with respect to each other. The magnetic sensor 103 is positioned in a homogeneous magnetic field of the rotating magnet 102 on a substrate 104.
In this rotational angle detection device, as the rotating shaft 101 rotates, the rotating magnet 102, which is attached to the rotating shaft 101, rotates accordingly, thereby providing a rotating magnetic field in the magnetic sensor 103. This would cause a change in resistance of each MR element 111 and change values of current, flowing through the bridge, to have a sinusoidal shape. Hence, the magnetic sensor 103 outputs a sinusoidal signal. A rotational angle of the rotating shaft 101 can be detected from that sinusoidal signal.
However, since the above-mentioned magnetic sensor 103, by its nature, has only an detectable angle range of 180°, from one easy-magnetization axis at the plus-end to the other easy-magnetization axis at the minus-end, it could not univocally detect an angle ranging from 0° to 360°.
For this purpose, a technology is known in the art that enables a rotational angle detection ranging from 0° to 360°, for example, by positioning a hall element in the vicinity of the magnetic sensor (as disclosed in Patent Document 2).
In addition, an angle sensor using a permanent magnet is known in the art as a device for reinforcing magnetic anisotropy in the magnetic sensor (as disclosed in Patent Document 3).
[Patent Document 1] Japanese Patent Laid-Open No. (HEI) 7-260414
[Patent Document 2] Japanese Patent Laid-Open No. (HEI) 11-94512
[Patent Document 3] Japanese Patent Laid-Open No. 2003-4480